Present day hack saws have a single blade with a hole on each end of the blade to attached it to the saw frame and handle. Hack saw blades have teeth that are generally for cutting metal and the blades are oriented to cut in only one direction, but may be used for cutting other materials with varying results.
Bow saws are used for cutting wood. The saw blade is a blade that has teeth that are formed to cut in either direction. Bow saw blades usually have a hole in each end for attaching the blade to the bow saw frame.
For both the hack saw and bow saw, the cutting is in the central portion of the blade since the ends of the blade are used for mounting the blade.